Process for increasing the corrosion-resistance of metals



he. July u, use

UNITED s'rares PATENT OFFICE 2,514,941 raocss's Foa nvcaeasmd m con- BOSION-RESISTANCE F METALS ltoyaton Fraser Dryadale and Ronald William Parker, Croydon, England, aasignors to The mterisation Company Limited, Croydon, Eng- No Drawing. Application July 26, 1941, Serial No. 104,029. in Great Britain May 1a, 1946 Section 1, Public Law s90, August s. 1940 Patent expires May 13, 1966 4 Claims. (Cl. 8-6.15)

l- This invention relates to improvements in processes for increasing the corrosion-resistance of ferrous and non-ferrous metals and alloys, particularly iron, steel, zinc and aluminium and its- The addition of such compounds, however, leads: to a lowering of the corrosion resistance of the formed coatings, and is thus contraindicated in cases where the protection of the metal is the primary object of the treatment. I

In U. 8. application Serial No. 821,171, now abandoned, it is disclosed that the formation of the phosphate coatings on metals and alloys is improved by the incorporation in the phosphating solution of one or more complex ammonlum phosphates of a divalent metal, characterised by the presence of at ieast'50% of the metal in the anion. The coating thus formed is of extremely fine grained uniform texture. and is highly resistant against corrosion.

It has now been found, according to the present invention that the formation of the phosphate coating on metals and alloys is greatly improved by the incorporation in the phosphating solution of a relatively small amount of one or more complex anhydrous ammonium metallophosphates and/or solid solutions of one such complex phosphate in another such complex phosphate, all such complex phosphates being characterised by the presence of all the metal in the anion. The coating thus formed is-of extremely fine grained uniform texture, highly resistant against corrosion and of greater density and lower porosity than normally produced phos phate coatings.

Anhydrous zinc ammonium phosphate is the only member of this group which has so far been isolated in the pure stateand is easily prepared by known means. The anhydrous ammonium phosphate of a number of other metals while unstable in the pure state are stabilized when precipitated in the form of solid solutions in the anhydrous zinc ammonium phosphate.

2 signed to anhydrous zinc ammonium phosphate z, Poi z. (Nmmrol 2.. Po. 2. (Bassett 8| Bedwell, J. 0. 8.1933, pages 854-882, inclusive). This substance is hereinafter referred to as the zinc complex. The general formula for such compounds is M Po. M

M P04 M where M is a divalent metal.

Solid solutions of divalent metal ammonium phosphates in zinc ammonium phosphate are conveniently prepared by coprecipitating a solution of salts of zinc and the other metal or -metals with diammonium hydrogen phosphate.

Analogous structural formulae have been assigned to these compounds.

In carryin the invention into practice the following compounds have been found extremely useful:

(a) The zinc complex (b) A solid solution of the cobalt complex 00 P0. O0 (NHOKPO; P04) in the zinc complex, subsequently referred to as the solid solution of the cobalt complex (0) A solid solution of the magnesium complex M (NH|)4(P04 Mg 1 0 1 in the zinc complex, subsequently referred to as the solid solution of magnesium complex.

(d) A solid solution of the cadmium complex Cd P04 Cd (NHOKP 4 0d P04 cu in the zinc complex, subsequently referred to as the cadmium complex.

In the preparation of these solid solutions the solubility of the unstable complex in the zinc complex is limited and any excess of the metal producing the unstable complex is precipitated as a hydrated phosphate. This does not seriously affect the etllciency of the product adversely however, since the hydrated complexes also have a beneficial effect on the phosphating bath. Furthermore some metals, for example, copper and nickel, do not form anhydrous ammonium phosphates, and therefore cannot be used in the present invention.

The preferred materials of the present inven-,

The structural formula which has been asll tion, namely, the zinc complex and the solid rust-proofing solution is prepared by dissolving in a concentrated aqueous solution of phosphoric acid, sulphide of iron and carbonate of soda.

For instance, 140 grammes of finely pulverized sulphide of iron are mixed with 40 grammes of carbonate of sodium. This mixture is dissolved in 1 litre of orthophosphoric acid, consisting of 400 cc. of acid having a concentration of 83% (density 1.7 or 60 Baum), and 600 cc. of water. The ingredients are allowed to remain in the receptacle until dissolution is complete. The solution is then filtered. The rustproofing bath is formed in a simple manner by mixing 30 cc. of the product with each litre of water. Approximately 3 kilograms of product are required to form a bath of 200 litres. It is not necessary to add any other ingredient to the bath during the rustproofing. The density of the bath thus obtained is 1.016, which is that of a solution of normal working strength.

The sodium carbonate can also be omitted entirely or can be added to the rustproofing bath at the time of its application. Analogous results are obtained by reacting phosphoric acid with other iron compounds, or with metallic iron, provided that there is added, either during the reaction or afterwards, a sulphide such as the sulphides of potassium, sodium and calcium. However, iron sulphide is preferable, because it has both the advantage of facilitating attack by the acid and of producing sulphuretted hydrogen. For instance, 336 grammes of iron sulphide are mixed with a litre of ortho-phosrhoric acid consisting of 500 cc. of 83% acid (density 1.7 or 60 Baum) and 500 cc. of water. The substances are allowed to remain together until the reaction is finished; the solution is then filtered. It can then be stored in suitable flasks or other vessels until used. A rustproofing bath is obtained by mixing the above product with water in the proportion of about 21 cc. the product per litre of water. In treating the articles to be protected, the bath is maintained at a temperature of 100 C. and the articles are left therein for about 1 /2 hours. The strength of the bath diminishes in proportion as the insoluble phosphate is deposited on the articles treated. A certain amount of the product is added from time to time in order to bring the bath to the composition desired. The amount of the product to be added can be determined easily by measuring the density of the bath.

To illustrate the practical application of the invention, there are described below four phosphating baths in which. the additives of the present invention areincorporated in the solutions described in British patent specifications Nos. 447,176 and 419,487:

I. 8 gms. oi the solid solution of the cobalt complex were dissolved in 100 mls. of concentrated H3PO4, and mls. of this solution were added to 1 litre of the concentrated solution described in British patent specifications Nos. 447,176 and 419,487. For use, the resultant con- 4 centrate was diluted to produce a bath of 30 points strength.

The points strength of the bath is the number of cc. of decinormal caustic soda required to neutralize 10 cc. of the bath solution using phenolphthalein as indicator.

II. gms. of the solid solution of the cobalt complex and 16 gms. of ammonium manganese phosphate monohydrate were dissolved in 450 mls. of concentrated H3PO4 and 10 mls. of this solution were added to 1 litre of the concentrated solution as in I.

III. 12 gms. of the solid solution of the magnesium complex and 16 gms. of ammonium nickelophosphate monohydrate were dissolved in 450 mls. of concentrate H3PO4 and 10 mls. of this solution added to 1 litre of concentrated solution, as in I above.

IV. 15 gms. of the solid solution of the cobalt complex and 16 gms. of ammonium nickelophosphate monohydrate were dissolved in 450 mls. of concentrated HaPO4 and 10 mls. of this solution were added to 1 litre of concentrated solution as in I above.

To establish the fact that the beneficial effect is due to the presence of the anhydrous complex, and not merely to the presence of its various components, and thus simultaneously establishing that the complexes do not decompose in solution in the bath, the following comparative tests were carried out:

Samples cut from the same steel sheet and treated in two baths of the same strength and temperature (Bath "A having additions specifled in IV above made thereto and Bath "B" having the same amount of ammonium nickelophosphate and the equivalent weight of hydrated ammonium cobaltophosphate dissolved in the same amount of concentrated H3PO4 added thereto) were compared. Results were as follows:

(1) Speed of phosphatation Measured by time to cessation of gassing. Bath A required only of time required by Bath B.

(2) Crystal size The major axes of the crystals constituting the phosphate coating were measured under high magnification. A large number of crystals were measured on each sample and the length of the smallest and largest crystal found on each speciment is quoted.

Bath A:

Shortest crystal 13 microns Largest crystal 28 microns Bath 3:

Shortest crystal 18 microns Largest crystal 34' microns (s) Uniformity of thickness of coating Transverse micro specimens were prepared from samples treated in the two baths and'the thickness of coating measured. The average thickness quoted is the mean of 40 thicknesses measured on each specimen.

Coating produced in solution A:

Average thickness 0.00061" Maximum thickness 0.00081" Minimum thickness 0.00045" Coating produced in solution B: Average thickness 0.00070" Maximum thickness 0.00090" Minimum thickness 0.00045" (4) Resistance to corrosion'by 3% sodium chloride solution which had passed into solution on the specimen treated in Bath Awas 58% less than from the specimen treated in Bath B.

The materials disclosed in the present specification are complex phosphates of divalent metals,

and it has been established that they are in fact anhydrous compounds characterised. by the presence 01 all the metal in the anion. It has also resistance of the coating. The amount of iron been discovered that complexes at ammonium phosphate with metals of othervalencies. such as aluminium or chromium give valuable results in phosphating baths. They may be prepared analogously to those already described. For example, boiling aqueous solutions 01 soluble aluminium and chromium salts (aluminium nitrate and chromium chloride) were poured into an excess of boiling aqueous dibasic ammonium phosphate, and the temperature was maintained at 90-95 C. until the precipitate crystallized. The resulting complex was filtered, washed with hot water and dried at 120 C. when incorporated in phosphating baths in the manner al.- ready described. improved results were obtained. No structure has, as yet, been attributed to these complex phosphates of trivalent metals. They are however considered to fall within the broad scope of the invention.

We claim:

1. A process for the productionot-a protective coating on the surface or a metal selected from the group consisting of iron, steel, zinc, and aluminum, which comprises treating said surface with a liquid coating composition containing an aqueous solution of a metal phosphate and phosphoric acid and containing also in dissolved state a. relatively small amount of anhydrous zinc ammonium phosphate.

2. A process for the production of a protective I coating on the surface oi a metal selected from aluminum, which comprises-treating said surface with a liquid coating composition containing an aqueous solution 0! a metal phosphate and phosphoric acid and containing also in dissolved state a relatively small amo'untof a solid solution of an anhydrous metallo-ammonium phosphate selected from the group consisting of anhydrous cobalt ammonium phosphate, anhydrous magnesium ammonium phosphate and anhydrous cadmium ammonium phosphate in solid anhydrous zinc ammonium phosphate.

3. A liquid coating composition for the production of a protective coating on the surface of a metal selected from the group consisting of iron, steel. zinc. and aluminum, said liquid coating composition containing an aqueous solution of a metal phosphate and phosphoric acid and containing also in dissolved state a relativeLv small amount of anhydrous zinc ammonium phosphate.

4. A liquid coating composition for the production oi a protective coating on the surface of a metal selected from the group consisting of. iron. steel, zinc, and aluminum, said liquid coating composition containing an aqueous solution or a metal phosphate and phosphoric acid and containing also in dissolved state a relatively small amount ot a solid solution of an anhydrous metallo-ammonium phosphate selected from the group consisting of anhydrous cobalt ammonium phosphate, anhydrous magnesium ammonium phosphate and anhydrous cadmium ammonium phosphate in solid anhydrous zinc ammonium phosphate.-

ROYSTON FRASER. DRYSDALE. RONALD WILLIAM PARKER.

REFERENCES CITED The following references are of record in the file or this patent: 

3. A LIQUID COATING COMPOSITION FOR THE PRODUCTION OF A PROTECTIVE COATING ON THE SURFACE OF A METAL SELECTED FROM THE GROUP CONSISTING OF IRON, STEEL, ZINC, AND ALUMINUM, SAID LIQUID COATING COMPOSITION CONTAINING AN AQUEOUS SOLUTION OF A METAL PHOSPHATE AND PHOSPHORIC ACID AND CONTAINING ALSO IN DISSOLVED STATE A RELATIVELY SMALL AMOUNT OF ANHYDROUS ZINC AMMONIUM PHOSPHATE. 